U.S. patent number 7,138,762 [Application Number 10/902,176] was granted by the patent office on 2006-11-21 for organic electroluminescent device and manufacturing method therefor.
This patent grant is currently assigned to Seiko Epson Corporation. Invention is credited to Katsuyuki Morii.
United States Patent |
7,138,762 |
Morii |
November 21, 2006 |
Organic electroluminescent device and manufacturing method
therefor
Abstract
An organic electroluminescent device includes an anode, a
light-emitting layer, and a cathode, which has a structure formed
by sequentially laminating, from the light-emitting layer side, a
first cathode formed of a material having a work function of 3.0 eV
or less, and a second cathode, formed of a material having a work
function higher than that of the first cathode, so that the total
thickness of the first and the second cathodes is 100 angstroms or
less. These elements are stacked on a substrate, and light is
emitted to an exterior of the device via at least the cathode.
Inventors: |
Morii; Katsuyuki (Suwa,
JP) |
Assignee: |
Seiko Epson Corporation (Tokyo,
JP)
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Family
ID: |
18596046 |
Appl.
No.: |
10/902,176 |
Filed: |
July 30, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050023968 A1 |
Feb 3, 2005 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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09810233 |
Mar 19, 2001 |
6853130 |
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Foreign Application Priority Data
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Mar 21, 2000 [JP] |
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2000-078664 |
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Current U.S.
Class: |
313/503; 313/506;
445/25; 445/24; 313/504; 313/498; 313/311 |
Current CPC
Class: |
H01L
51/5231 (20130101); G02F 1/133603 (20130101); H01L
51/5246 (20130101); H01L 51/5253 (20130101) |
Current International
Class: |
H01J
9/26 (20060101); H01J 1/62 (20060101); H01J
9/00 (20060101) |
Field of
Search: |
;313/503 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Gu, G. et al., "Transparent organic light emitting devices," Appl.
Phys. Lett., vol. 68, No. 19, May 6, 1996, pp. 2606-2608. cited by
other.
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Primary Examiner: Patel; Nimeshkumar D.
Assistant Examiner: Walford; Natalie K.
Attorney, Agent or Firm: Oliff & Berridge, PLC
Parent Case Text
This is a Division of application Ser. No. 09/810,233 filed Mar.
19, 2001 now U.S. Pat. No. 6,853,130, which claims the benefit of
Japanese Patent No. 2000-078664 filed Mar. 21, 2000. The entire
disclosure of the prior application is hereby incorporated by
reference herein in its entirety.
Claims
What is claimed is:
1. An organic electroluminescent device comprising: a substrate; an
anode; a light-emitting layer formed of an organic material; a
cathode including a first cathode formed of a material having a
work function of 3.0 eV or less and a second cathode formed of a
material having a work function higher than the work function of
the first cathode, the first and second cathodes being sequentially
stacked in this order from the light-emitting layer, and light
being emitted to the outside via at least the cathode; and a
sealing layer formed on the cathode, the sealing layer having a
first sealing layer and a second sealing layer, the first sealing
layer being formed of LiF, and the thickness of the first sealing
layer being more than 300 angstrom and less than 500 angstrom.
2. The organic electroluminescent device according to claim 1, a
total thickness of the first and the second cathodes being 100
angstroms or less.
3. The organic electroluminescent device according to claim 1, the
first cathode including Ca or Au.
4. The organic electroluminescent device according to claim 1, the
thickness y (angstrom) of the first cathode being such that
50.ltoreq.y.ltoreq.80.
5. The organic electroluminescent device according to claim 1, the
thickness y (angstrom) of the first cathode being such that
55.ltoreq.y.ltoreq.65.
6. The organic electroluminescent device according to claim 1, the
second cathode including Al.
7. The organic electroluminescent device according to claim 1, the
thickness z (angstrom) of the second cathode being such that
10.ltoreq.z.ltoreq.20.
8. The organic electroluminescent device according to claim 1, the
organic material forming the light-emitting layer being a polymeric
material.
9. A method for manufacturing an organic electroluminescent device,
comprising: forming an anode on a substrate; forming a
light-emitting layer formed of an organic material above the anode;
forming a cathode above the light-emitting layer by laminating a
first cathode formed of a material having a work function of 3.0 eV
or less and a second cathode formed of a material having a work
function higher than that of the first cathode in this order; and
forming a sealing layer on the cathode by laminating a first
sealing layer formed of LiF and a second sealing layer in this
order, a thickness of the first sealing layer being more than 300
angstrom and less than 500 angstrom.
10. The method for manufacturing an organic electroluminescent
device according to claim 9, wherein: the forming of the cathode
above the light-emitting layer is performed using a vacuum
deposition method, and the forming of the first sealing layer on
the cathode is performed using a vacuum deposition method.
11. The method for manufacturing an organic electroluminescent
device according to claim 10, wherein the forming of the cathode
and the first sealing layer is performed using vacuum deposition
method without being exposed to air.
Description
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to organic electroluminescent
devices, which are electrical device light emitting devices used
for displays, light sources for displays, and the like, and to
manufacturing methods therefor.
2. Description of Related Art
Recently, as self-luminous displays replace liquid crystal
displays, development has been advancing rapidly of light-emitting
devices (organic electroluminescent devices, hereinafter referred
to as "organic EL devices"), which have a structure in which a
light-emitting layer formed of an organic material is provided
between an anode and a cathode. Among these, a high-transmission EL
device, a so-called transparent EL device (TOELD) used in the
visible light region, which can emit light from the two electrode
sides, has been desired, since overlapped displays can be performed
by disposing another display device thereunder. A particular
structure of this device is disclosed in, for example, Appl. Phys.
Lett. 68(19), 6 May 1996, p 2606. In this publication, it is
disclosed that an aluminum complex Alq3, which is a low molecular
material, is used as a light-emitting layer, a cathode is formed by
co-deposition of Mg and Ag, an ITO film is formed by sputtering
thereon for sealing or for assistance to the cathode so as to form
a device, and a threshold voltage of approximately 8 V is achieved.
In the structure described above, in view of the life and the
threshold characteristics of a material used for the light-emitting
layer, Mg and Ag are used for the cathode, and in addition, ITO is
used for the upper layer thereon.
Concerning organic EL devices, a light-emitting material having a
low threshold value can be used for a light-emitting layer, and a
metal material having a low work function can be used for a cathode
so as to realize operation at a low threshold voltage. However, in
the structure proposed in the paper described above, Mg and Ag are
not sufficient in view of the work functions, and since ITO is
additionally deposited on the metal material by sputtering, in
order to prevent the degradation thereof, Mg is oxidized, whereby a
problem may arise in that an increase in threshold voltage of the
device cannot be finally avoided.
SUMMARY OF THE INVENTION
The present invention addresses the above problem, and an object of
the present invention is to provide an organic electroluminescent
device, which can be operated at a low voltage, and which has high
efficiency, high transmission characteristics, and a long life. It
is also an object of the invention to provide a manufacturing
method therefor.
According to the present invention, an organic EL device is
provided, which includes an anode, a light-emitting layer formed of
an organic material, and a cathode, which has a structure in which
a first cathode, formed of a material having a work function of 3.0
eV or less, and a second cathode, formed of a material having a
work function higher than that of the first cathode, are
sequentially stacked from the light-emitting layer side, and the
total thickness of the first and the second cathodes, being 100
angstroms or less, are stacked on a substrate, and light is emitted
to the exterior of the device via at least the cathode.
In addition, according to the present invention, a method for
manufacturing an organic electroluminescent device is provided,
which includes the steps of: forming an anode on a substrate;
forming a light-emitting layer formed of an organic material above
the anode; and forming a cathode above the light-emitting layer by
laminating a first cathode, formed of a material having a work
function of 3.0 eV or less, and a second cathode, formed of a
material having a work function higher than that of the first
cathode, from the light-emitting layer side, so that the total
thickness of the first and the second cathodes is 100 angstroms or
less.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view showing a device structure of an
organic EL device according to an embodiment of the present
invention;
FIG. 2 is a graph showing a transmission spectrum in the visible
light region of an organic EL device formed in Example 1 of the
present invention;
FIG. 3 is a graph of the transmission spectrum showing time
dependence of a change in transmittance of a device processed by an
oxygen plasma treatment in an example of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
An organic EL device of the present invention includes an anode and
a light-emitting layer formed of an organic material provided on a
substrate. A cathode is provided, which has a structure in which a
first cathode, formed of a material having a work function of 3.0
eV or less, and a second cathode, formed of a material having a
work function higher than that of the first cathode, are
sequentially stacked with each other from the light-emitting layer
side. The total thickness of the first and the second cathodes is
100 angstroms or less. Light is emitted to the outside via at least
the cathode.
Preferred features of the organic EL device include the following:
(1) In the organic EL device, the cathode side is sealed by a
sealing layer formed of a transmissive material. (2) In the organic
EL device, the first cathode includes Ca. (3) In the organic EL
device, the thickness y (angstrom) of the first cathode is such
that 50.ltoreq.y.ltoreq.80 holds. (4) In the organic EL device, the
thickness y (angstrom) of the first cathode is such that
55.ltoreq.y.ltoreq.65 holds. (5) In the organic EL device, the
second cathode includes Al. (6) In the organic EL device, the
thickness z (angstrom) of the second cathode is such that
10.ltoreq.z.ltoreq.20 holds. (7) In the organic EL device, an
organic material forming the light-emitting layer is a polymeric
material.
In addition, a method for manufacturing an organic EL device of the
present invention includes the steps of forming an anode on a
substrate; forming a light-emitting layer formed of an organic
material above the anode; and forming a cathode above the
light-emitting layer by laminating a first cathode, formed of a
material having a work function of 3.0 eV or less, and a second
cathode, formed of a material having a work function higher than
that of the first cathode, from the light-emitting layer side, so
that the total thickness of the first and the second cathodes is
100 angstroms or less.
Preferred features of the method for manufacturing the organic EL
device include the following: (8) In the step of forming the anode
in the method for manufacturing the organic EL device, after an
electrode film is formed, an oxygen or an air plasma treatment is
performed under conditions in which a current x and a time t are
set such that 10 (mA).ltoreq.x.ltoreq.15 (mA) and 5
(minute).ltoreq.t.ltoreq.7 (minute) hold. (9) In the step of
forming the anode in the method for manufacturing the organic EL
device, after an electrode film is formed, an oxygen or an air
plasma treatment is performed under conditions in which a current x
and a time t are set such that 10 (mA).ltoreq.x.ltoreq.12 (mA) and
t=5 (minutes) hold.
Hereinafter, the embodiments of the present invention will be
described in detail with reference to drawings.
FIG. 1 is a cross-sectional view showing the structure of an
organic EL device according to the present invention.
In the structure shown in FIG. 1, the following elements are
stacked on a substrate 1: an anode 2, a hole injection/transport
layer 3, a light-emitting layer 4 formed of an organic material, a
first cathode layer 5 formed of a material having a work function
of 3.0 eV or less, and a second cathode 6 (a cathode is formed of a
stacked structure of the first and the second cathodes mentioned
above) formed of a material having a work function higher than that
of the first cathode. Next, the stacked structure described above
is sealed by a first sealing layer 7, and a second insulating layer
8, and in addition, is sealed by a sealing substrate 9.
As the substrate 1, a transparent material, such as glass, or a
reflective material, may be used. When the transparent material is
used, light can be emitted to the outside via at least the
substrate 1.
As a material used for the anode, for example, ITO or IDIXO
(manufactured by Idemitsu Kosan Co., Ltd.) may be used as a
transparent electrode material. The material mentioned above is
deposited on a substrate so as to form an electrode by sputtering
or the like. The transparent electrode thus formed is preferably
processed by an oxygen plasma treatment or an air plasma treatment
after the material is deposited.
The plasma treatment mentioned above is preferably performed at a
current of 10 mA to 15 mA for 5 minutes to 7 minutes as a treatment
time, and is more preferably performed at a current of 10 mA to 12
mA for 5 minutes. When the treatment time is less than 5 minutes,
in particular, the transmittance of the wavelength in the range of
400 to 550 nm is decreased by up to 3 to 5%, compared to that in
the case in which the treatment time is more than 5 minutes. In
contrast, from 5 minutes to 10 minutes for the treatment, an
increase in transmittance in the region of the wavelength mentioned
above is not observed. For example, when an oxygen plasma treatment
is performed, it is preferable that VPS020 manufactured by Sanyu
Electron Co., Ltd. be used, and that the treatment be performed
after purging is performed 2 to 3 times by using oxygen.
In addition, when the current is less than 10 mA, the uniformity of
surface treatment may be degraded in some cases, and when the
current is more than 15 mA, a decrease in film thickness may occur
by ashing in some cases. In addition, a decrease in transmittance
of the wavelength in the range of 600 to 800 nm is observed by the
treatment described above in accordance with the treatment time.
Compared to the case in which the treatment is not performed, when
the treatment is performed for 10 minutes, a decrease in
transmittance by up to approximately 2% is observed. This may occur
due to a change in band structure caused by oxidation of the
surface of the transparent electrode, and by the generation of
defects therein due to the treatment described above. Accordingly,
an oxygen plasma treatment or an air plasma treatment is preferably
performed approximately at 10 mA for 5 minutes, and as a result, an
increase in transmittance in the vicinity of 500 nm, to which
humans are more sensitive, and an increase in transmittance on
average in the visible light region can be realized.
In this connection, when the transparent electrode material
described above is used for the anode 2, light can be emitted to
the exterior of the device via at least the substrate 1.
In addition, as the anode 2, for example, a layer formed of a
metal, such as Pt, Ir, Ni, Pd, or Au, or a stacked structure of a
transparent material layer formed of ITO or the like and a
reflective layer formed of Al or the like may be used.
Furthermore, the substrate provided with the anode 2 formed thereon
is preferably an active matrix substrate having a plurality of
anodes disposed thereon and switching devices, such as thin-film
transistors, each provided for each anode.
In this embodiment, the hole injection/transport layer 3 is
provided between the anode 2 and the light-emitting layer 4. In
this connection, the hole injection/transport layer 3 is a layer
having a function of hole injection or hole transfer from the anode
2 to the light-emitting layer side. For the hole
injection/transport layer 3 described above, a mixture of
polyethylenedioxy-thiophene
##STR00001## and polystyrene sulfonate,
##STR00002## copper phthalocyanine, or the like is preferably
used.
For the light-emitting layer 4, a low molecular weight organic
light-emitting material or a polymeric light-emitting material may
be used, a fluorene-based polymer is preferably used, and in
particular,
##STR00003## polymeric organic material, such as PPV
(poly-p-phenylenevinylene) is preferably used.
For the first cathode 5, as described above, a material having a
work function of 3.0 eV or less is used. As the cathode material
mentioned above, in particular, Ca is preferably used.
Specifically, Ca is preferable since a low threshold voltage,
because of the low work function, and a high transmittance, because
of the low reflectance for visible light, can be realized. The
thickness of the first cathode described above is preferably from
50 to 80 angstroms, and more preferably, from 55 to 65 angstroms.
When the thickness is less than 50 angstroms, due to the influence
of the work function of the second cathode 6, which is an upper
layer, there may be a risk in that the threshold voltage of the
device is increased. In addition, when it is 80 angstroms or more,
there may be a risk in that the transmittance is significantly
decreased.
In particular, in the case in which Ca is used, since Ca has
absorption over almost the entire visible wavelength region, when
the thickness thereof is excessively large, black tone becomes
significant throughout the cathode side. In particular, when the Ca
film has a thickness of approximately 80 angstroms, it is believed
that a continuous film is formed having a certain level of
thickness such that electrical conductance can be achieved. From
this point of view, it is also preferable that the thickness of the
first cathode 5 be 80 angstroms or less. In addition, as the
cathode, Au may also be used.
The first cathode 5 described above can be formed by vacuum
deposition at a degree of vacuum of, for example, 1.times.10.sup.-6
torr or more.
In addition, for the second cathode 6, a material having a work
function higher than that of the first cathode 5 is used. As a
material therefor, a material, which has a work function not
significantly higher than that of a material for the first cathode
5, has a certain level of stability to oxygen, and can easily form
a continuous film, is preferably used. In particular, Al and Ag may
be used.
In particular, when Ca is used for the first cathode 5, it is
preferable that Al or the like be used as a material for the second
cathode 6. The thickness of the second cathode 6 described above is
preferably set to be 10 to 20 angstroms, and more preferably, is
set to be 10 angstroms. When it is less than 10 angstroms,
electrical conductance cannot be obtained, and in addition, when it
is more than 20 angstroms, there may be a risk in that the
transmittance is significantly decreased by metal reflection of the
material (particularly, a metal material) itself for the second
cathode 6.
In the present invention, as described above, by forming a layer
formed of a material having a low work function (3.0 eV or less) as
the first cathode 5 and by forming a layer, which is a continuous
layer and has a work function higher than that described above, on
the first cathode 5 as the second cathode 6, the degradation of the
first cathode 5 is prevented. In addition, since the total
thickness of the first cathode 5 and the second cathode 6, which
form the stacked structure, is formed so as to be 100 angstroms or
less, the light transmission characteristics at the cathode side is
ensured.
As a method for forming the stacked structure of the cathodes
described above, it is preferable that after the first cathode 5 is
formed by deposition, it be confirmed that the degree of vacuum
reaches a level approximately equivalent to that at which the first
cathode 5 is deposited, and the second cathode 6 be then formed
under conditions similar to those for the first cathode 5.
As the first sealing layer 7, for example, LiF, SiO, or SiO.sub.2
is used. In particular, since a film of LiF can be easily formed by
a vacuum deposition method, the device can be formed without being
exposed in the air from the film formation of the cathode to a
subsequent series of operations in an inert atmosphere, and in
addition, since the material itself contains no oxygen atom,
conditions containing oxygen at a nearly zero concentration thereof
can be maintained. Furthermore, the transmittance in the visible
light region is also high, and the transmission characteristics are
not degraded. The thickness and the deposition rate are set to be
300 to 500 angstroms and 8 angstroms/sec or more, respectively.
When the thickness is less than 300 angstroms, it is difficult to
protect the cathode, which is a lower layer, against water and
oxygen from the outside air and the infiltration of water and air
from the second sealing layer 8, which is an upper layer, by
sealing. In addition, when the thickness of the film is more than
500 angstroms, the device (in many cases, light-emitting layer) is
damaged by heat radiation during deposition, and hence, there may
be a risk in that the intrinsic EL light-emitting characteristics
are damaged. Furthermore, when the deposition rate is 8
angstroms/sec, since the deposition time is long, device
degradation also occurs by heat radiation as is the case described
above. As a result, the deposition rate is also limited, and hence,
it is required to have 8 angstroms/sec or more.
As the second sealing layer 8, for example, a transparent heat
curable epoxy resin or a photocurable epoxy resin is used. In
particular, a heat curable epoxy resin is preferable, and more
particularly, coating thereof is performed by dipping, a glass
substrate, which is the sealing substrate 9, is placed thereon, and
curing is then performed in an inert atmosphere, thereby forming
the sealing layer. As the epoxy resin, a moisture-proof resin, such
as DPpure60 (manufactured by 3M), or STYCAST1269A (Emeron), may be
used.
Hereinafter, the present invention will be described in detail with
reference to examples.
EXAMPLE 1
An organic EL device having a structure shown in FIG. 1 was
formed.
Operations described below were all performed in a clean room.
On a washed glass substrate 1 of 150 mm square, an (transparent)
electrode (anode) 2 (IDIXO) having 1,000 angstroms thick was formed
by sputtering. The conditions therefor were: a degree of vacuum of
1.times.10.sup.-4 Pa or less, an Ar to O.sub.2 flow ratio of 10:1,
320 V, 0.15 mA, and 14 minutes. Next, an oxygen plasma treatment
was performed on the anode film formed on the glass substrate at a
current of 10 mA for 5 minutes. In particular, VPS020 manufactured
by Sanyu Electron Co., Ltd. was used, and the treatment was
performed after purging were performed 2 to 3 times using
oxygen.
Subsequent operations described below were performed in a closed
container. The conditions in the closed container were such that
the oxygen concentration was 0.01 ppm or less, and a dew point of
water was -70.degree. C. or less.
First, a mixture of PEDOT (polyethylenedioxy-thiophene) and PSS
(polystyrene sulfonate) was applied as a hole injection/transport
material on the electrode 2 processed by the plasma treatment
described above. The mixture described above could be obtained from
Bayer A.G as Baytron P. In this example, a solution was formed by
mixing Baytron P with PSS in a ratio of 5:1 and by diluting the
mixture with water to 1.5 times, and by using the solution thus
obtained, a film was formed by spin coating. Under conditions, such
as a slope of 1 second, 3,000 revolutions, and 45 seconds, a film
600 angstroms thick was formed. By firing the film at 200.degree.
C. for 10 minutes, film formation was performed, thereby yielding a
hole injection/transport layer 3.
Next, on the hole injection/transport layer 3, a solution of a
fluorene-based polymer having a structure shown below dissolved in
a xylene solvent was applied by spin coating so as to have a film
thickness of 800 angstroms, thereby forming a light-emitting layer
4.
##STR00004##
Subsequently, on the light-emitting layer 4, first, a film
formation (deposition) of Ca was performed in a vacuum deposition
apparatus so as to form a first cathode 5. The vacuum deposition
apparatus disposed in the closed container was used. The degree of
vacuum at the beginning was approximately 1.times.10.sup.-6 torr.
The deposition rate was set to be 3 angstroms/sec, and the film
thickness was set to be 70 angstroms. Next, after the degree of
vacuum again reaches 1.times.10.sup.-6 torr, a film 10 having
angstroms thick was formed as the second cathode 6 by depositing Al
at a deposition rate of 3 angstroms/sec.
Next, on the second cathode 6, as a first sealing layer 7, a film
having 500 angstroms thick was formed by depositing LiF at a
deposition rate of 8 angstroms/sec.
After cooling, on the first sealing layer 7, DPpure60 (manufactured
by 3M) was applied (film thickness, 200 .mu.m) which was a
moisture-proof epoxy resin, thereby forming a second sealing layer
8. A sealing glass (thickness, 0.3 mm) was then adhered thereto as
a sealing substrate 9, and compressing was performed by a hot plate
under conditions, at 50.degree. C. for 12 hours, whereby curing was
performed. In this step, in order to remove air bubbles in the
epoxy resin, heating was performed at a degree of vacuum of
approximately 0.1 torr. As a result, an organic EL device was
obtained.
For the organic EL device thus obtained, the transmittance and the
threshold voltage were measured. The measurement of the
transmittance was performed by using a spectroscope (manufactured
by Hitachi, Ltd.), in which air was used as the base line, and a
pinhole of 3 mm in diameter was provided at a condenser portion.
The measurement of the threshold voltage was performed using BM-7
(manufactured by Kabushiki Kaisha Topcon), and the threshold
voltage was defined by a voltage at which a luminance of 5
Cd/m.sup.2 is output.
The results are shown in FIG. 2.
According to the results shown in FIG. 2, it was understood that
the transmittance was 50% or more over almost the entire visible
light region. In consideration of the glass substrate 1 having a
thickness of 1.1 mm, and the transmittance thereof being
approximately 75%, it was believed that a transmittance of 70% or
more was achieved. In the case described above, the threshold
voltage was 3 V.
EXAMPLE 2
In accordance with the method described in Example 1, glass
substrates, each having an anode stacked thereon, were obtained,
wherein the anodes formed of transparent electrodes were processed
by an oxygen plasma treatment for from 0 to 10 minutes at
one-minute intervals. For glass substrates having anodes thereon,
which were processed for 0, 5 (Example), and 10 minutes,
transmission spectrums (transmittance at each wavelength) were
measured. The results are shown in FIG. 3. In addition, for the
glass substrates having anodes thereon, which were processed for
from 0 to 10 minutes at one-minute intervals, transmittances at 450
nm, 550 nm, and 700 nm were measured. The results are shown in
Table 1.
TABLE-US-00001 TABLE 1 Time for an Oxygen Plasma Treatment 1 2 3 4
5 6 7 8 9 10 0 (min) (min) (min) (min) (min) (min) (min) (min)
(min) (min) (min) Transmittance 75.2 75.8 76.3 77 77.5 78.2 78.3
78.2 78.1 78.2 78.1 (%; at 450 nm) Transmittance 74.6 74.7 74.9 75
75.2 75.2 75.3 75 74.8 74.5 74.3 (%; at 550 nm) Transmittance 75.8
75.7 75.5 75.5 75.3 75.2 75 74.9 74.6 74.4 74.3 (%; at 700 nm)
In the blue region (450 nm), an increase in transmittance was
observed until 5 minutes elapse from the start of the treatment. In
the green region (550 nm), even though a significant change was not
observed, the transmittance exhibited the maximum value
approximately 5 minutes after the start of the treatment. In the
red region (700 nm), as the treatment time elapsed, the
transmittance was decreased even though this decrease was not
significant. It is believed that the phenomenon described above
occurred by a change of the band structure, since the material was
partly changed due to the oxidation of the surface thereof by the
treatment. Accordingly, it was considered that a treatment time of
approximately 5 minutes was an optimum condition.
EXAMPLE 3
Organic EL devices were formed in a manner similar to that in
Example 1, except that the first cathodes were formed so as to have
thicknesses of 40, 50, 60, 80, 90, and 100 angstroms. The threshold
voltages and the transmittances at a wavelength of 550 nm were
measured for the individual organic EL devices. In Table 2, the
results of the threshold voltages and the transmittances (a
wavelength region of 550 nm) are shown together with the results of
Example 1 (the first cathode having a film thickness of 70
angstroms).
TABLE-US-00002 TABLE 2 Film Thickness of First 50 60 70 80 90 100
Cathode 40 (.ANG.) (.ANG.) (.ANG.) (.ANG.) (.ANG.) (.ANG.) (.ANG.)
Threshold 5 3.5 3.5 3 2.9 2.9 2.9 voltage (V) Transmittance 61 59
56 54 50 45 38 (%, at 550 nm)
Concerning the threshold voltage, it was asymptomatically
stabilized at a film thickness of the first cathode of
approximately 70 angstroms. An increase in threshold voltage was
observed when the film thickness was not more than 70 angstroms. It
is believed that the phenomenon described above occurred by an
increase in resistance caused by the influence of the work function
of the second cathode, or by the insufficient film thickness of the
second cathode.
The transmittance was decreased with the change in film thickness
of the first cathode, the acceptable level was up to approximately
90 angstroms (a total thickness of the first and the second
cathodes of 100 angstroms), and at a level of the thickness of the
first cathode higher than that mentioned above (a level higher than
that of a total thickness of the first and the second cathodes of
100 angstroms), the transmittance was excessively decreased.
According to the results described above, it was believed that a
film thickness of approximately 70 angstroms of the first cathode
was an optimum value.
EXAMPLE 4
Organic EL devices were formed in a manner similar to that in
Example 1, except that the second cathodes were formed so as to
have thicknesses of 5, 15, 20, 25, and 40 angstroms. The threshold
voltages and the transmittances at a wavelength of 550 nm were
measured for the individual organic EL devices. In Table 3, the
results of the threshold voltages and the transmittances are shown
together with the results of Example 1 (the second cathode having a
film thickness of 10 angstroms).
TABLE-US-00003 TABLE 3 Film Thickness of Second Cathode 10 15 20 25
40 5 (.ANG.) (.ANG.) (.ANG.) (.ANG.) (.ANG.) (.ANG.) Threshold
voltage (V) No Light Emission 3 3.1 3 3 3 Transmittance (%, at 57
54 49 43 32 20 550 nm)
Concerning the threshold voltage, when the thickness thereof was 10
angstroms or more, the change could not be observed. When the
thickness was 10 angstroms or less, it is believed that light
emission was not performed since conductance could not be obtained.
Since a constant value was obtained, it was believed that there was
no influence of the work function of the second cathode. A decrease
in transmittance was significant compared to the case in which Ca
was used. It is believed that the phenomenon was caused by Al,
which had high conductivity and high reflectance.
As has been thus described in detail, according to the present
invention, the transmittance in the visible region is enhanced,
sealing defects are reduced, and the influence of oxygen, water,
and the like in the outside air can be minimized as much as
possible. In addition, as a transmissive type organic EL device, a
device can be realized which has a long life, is driven at a low
voltage, and has a high transmittance in the visible light
region.
* * * * *